CN112858970A - Method and system for compensating for stray magnetic fields in a magnetic resonance imaging system - Google Patents

Method and system for compensating for stray magnetic fields in a magnetic resonance imaging system Download PDF

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CN112858970A
CN112858970A CN202011342357.XA CN202011342357A CN112858970A CN 112858970 A CN112858970 A CN 112858970A CN 202011342357 A CN202011342357 A CN 202011342357A CN 112858970 A CN112858970 A CN 112858970A
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斯特凡·波佩斯库
马库斯·韦斯特
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Abstract

The invention relates to a method and a system for compensating for stray magnetic fields in a magnetic resonance imaging system. The invention describes a method for compensating for a stray magnetic field (SF) in a magnetic resonance imaging system (1) having two or more examination zones (M1, M2, M3, M4, M5, M6), comprising: providing for a predefined first magnetic field (G) to be applied in a first examination zone (M1) in addition to the basic magnetic field (B0)Z1) A value of (d); providing a predefined sequence of control pulses (G) defining a second examination zone (M2)Z2) The information of (a); for applying a first magnetic field (G) in a first examination zone (M1)Z1) In the case of (2), determining a stray in the second examination region (M2)A magnetic field (SF); controlling the pulses (G) according to a predefined sequenceZ2) And the determined magnetic Stray Field (SF) to calculate a compensation sequence control pulse (CG) for the second examination zone (M2); a compensation sequence control pulse (CG) is applied to the second examination region (M2).

Description

Method and system for compensating for stray magnetic fields in a magnetic resonance imaging system
Technical Field
The present invention describes a method and a system for compensating for stray magnetic fields in a magnetic resonance imaging system ("MRI-system") having two or more examination zones, and such an MRI system.
Background
For over forty years, the principles of magnetic resonance imaging ("MRI") have been used for imaging and other measurements. Despite the long and important time of this measurement method, only two magnet designs are currently used for clinically used MRI systems or MRI scanners: c magnet form and solenoid. The operation of this type of MRI scanner remains problematic for clinical workflow.
The most serious problem is the wide stray magnetic field around these scanners. To deal with this problem and avoid accidents and injuries, hospital authorities must delimit strictly controlled areas inside and near MRI examination rooms by restricting the access of people and equipment. Damage may occur if a metal or magnetic component is attracted by the strong magnet of the MRI scanner and accelerates in the direction of the scanner volume.
Another problem is that MRI scanners using a solenoid-magnet design "wrap" the patient in a narrow patient tunnel, which can lead to claustrophobia, among other things. Claustrophobia may be so intense in some patients that MRI scanning is not possible. Furthermore, access to the patient by medical staff is severely restricted due to examination of tunnel stenosis, which is disadvantageous for interventional or therapeutic procedures, especially for real-time MRI imaging.
Generally, MRI scanners use self-shielded solenoidal superconducting magnets to reduce the strength of the leakage magnetic field generated by the coils of the basic field magnet. An actively shielded basic field magnet is much more expensive than an unshielded basic field magnet. In addition, the shield coil reduces the efficiency of the basic magnetic field that can be used for inspection measurements in the tunnel. The active bucked magnet has a larger diameter (about 220cm) than the unshielded magnet (about 145 cm).
An alternative design of MR scanner uses C-shaped magnets. This may be an electromagnet comprising two helmholtz coils or a permanent magnet. The C-shaped magnet has two pole pieces which generate a perpendicular basic magnetic field in their space. A similar structure is a gate magnet, which is mechanically stronger and may also be implemented with superconducting helmholtz coils in some embodiments. The C-shaped magnet and the door magnet have the advantage of being freely accessible to the patient and additionally reduce the sense of claustrophobia. However, such a structure requires a very robust mechanical construction to counteract the large magnetic attraction between the two opposing basic field magnets. To reduce the propagation of stray magnetic fields, these magnet architectures typically use iron yokes to direct magnetic field lines outside the imaging volume. The iron yoke is one of the most cost effective shields. A disadvantage of such an iron yoke is the large size, weight and volume of the MR scanner.
Approaches to address these problems have been introduced shortly before. The method is based on an MRI system with a toroidal magnetic field. Unlike prior art MR magnets that use solenoid or Helmholtz-pair (Helmholtz-pair) magnet coils, toroidal coils tend to confine the magnetic field inside the toroid with only small and less far stray magnetic fields. This system not only overcomes the problems of stray magnetic fields and light weight construction, but also provides the opportunity to achieve two or more examination regions in one single MRI system. Examples of such MRI systems are basic field magnet arrangements with three, four, six or eight (e.g. identical) basic field magnet segments arranged in a star shape (e.g. 60 ° for six magnets and six examination zones) rotationally symmetrically around a central axis. The basic magnetic field has a main direction which extends in the form of a toroidal magnetic field.
There are local gradient systems with coil pairs arranged parallel to the left and right side of the patient. However, although this known gradient system can also be used for these new MRI systems, there is currently no gradient system that works in an optimal way with these MRI systems.
In MRI systems with more than one examination zone, gradient systems are installed in the examination zone, and stray gradient fields occur in other examination zones. In case each examination region comprises a gradient system, there are stray gradient fields over the entire examination region. These stray gradient fields negatively affect the examination region. Therefore, it is desirable to shield stray gradient fields in neighboring regions very well, since otherwise even small asynchronous perturbations, even of the order of about 1ppm, may have produced image artifacts.
Typical actively shielded gradient coils reduce stray fields by only an order of magnitude. This is sufficient to reduce the magnitude of eddy currents induced into the conductive portions of the magnet, but is not sufficient to effectively shield stray magnetic fields in the adjacent examination region. In MRI systems with two or more examination zones, simultaneous but asynchronous imaging requires shielding between the individual gradient systems, which is orders of magnitude better than in the prior art.
Disclosure of Invention
It is an object of the invention to improve the known MRI system with more than one examination zone to facilitate improved measurements by compensating for stray gradient fields. This object is achieved by a method, a system, a control device and a magnetic resonance imaging system according to the invention.
The method according to the invention for compensating for stray magnetic fields in a magnetic resonance imaging system having two or more examination zones comprises the steps of:
providing, in addition to the basic magnetic field, a value of a predefined first magnetic field to be applied in the first examination zone,
providing a predefined sequence of control pulses to be applied in a second examination region (in particular adjacent to the first examination region, since the effect is strongest in the adjacent region),
determining stray magnetic fields in the second examination zone in case a first magnetic field is applied in the first examination zone,
calculating a compensation sequence control pulse for the second examination zone from the predefined sequence control pulse and the determined stray magnetic field, wherein the compensation sequence control pulse is calculated such that measurements in the second examination zone can be performed irrespective of the stray magnetic field,
-applying a compensation sequence control pulse to the second examination region, and
preferably, these steps are repeated for other examination zones, in particular for all examination zones.
The first magnetic field is not a basic magnetic field, because it should be applied in addition to the basic magnetic field. Preferably it is a gradient field, but it can also be another magnetic field, for example a shim or the field of an active shielding device. The value for this first magnetic field applied in the first examination zone is known. When applied in a first examination zone, the first magnetic field generates a stray field in the other examination zones.
The stray field influences the measurement in the second examination region. If the second examination zone is adjacent to the first examination zone (wherein this is preferred because the stray field is strongest in the adjacent zone), the stray magnetic field will disturb the measurement seriously. For the measurement, a predefined sequence of control pulses is applied in the second examination zone, wherein the predefined sequence of control pulses is preferably a predefined second magnetic field (in particular a gradient field) or a predefined RF signal. Since the stray field affects the measurement with the sequence of control pulses, the sequence of control pulses is adjusted to the stray field with the following steps.
The information defining the predefined sequence of control pulses is data about the strength and direction of the sequence of control pulses. Since there are defined magnet coils and RF coils/antennas in an MRI system, the data may include information about the signal amplitude or current and the antenna or coil to which the signal is applied.
It should be noted that in all examination regions of the MRI system, the influence of stray magnetic fields should be compensated for. Thus, preferably, the values of the predefined sequence of control pulses of all examination regions should be provided and the method should be performed for all examination regions while considering any examination region as a first region and any other examination region as a second examination region.
Before, during or after any information on the predefined sequence of control pulses is provided, the stray magnetic fields in the second examination zone are determined, for example, their direction and their strength (magnetic field vector). This is the stray field of the first magnetic field. This step can be achieved by calculation or by measuring the stray magnetic field.
For example, a first magnetic field may be applied in a first examination region, and stray magnetic fields (e.g., for different currents inducing the first magnetic field) may be measured in a second examination region. For the case of an application of a first magnetic field (with a predefined current) in the first examination zone, this measurement value can be stored and used for determining the stray magnetic field in the second examination zone. However, if the properties of the MRI scanner are well known, the magnetic field may also be calculated (e.g. in simulations). Finally, for a set of identical MRI scanners, a set of stored values may be used to make the determination.
Using the determined stray magnetic field and the provided (predefined) sequence control pulse, a compensation sequence control pulse can be calculated for the second examination zone. The compensation sequence control pulse may be determined directly or a correction term may be calculated and added to/subtracted from the predefined sequence control pulse. Since the direction of the predefined sequence of control pulses and the stray field may be important, the resulting compensation vector is preferably calculated from the vector representing the predefined sequence of control pulses and the correction vector (based on the stray field).
Thereafter, a compensation sequence control pulse is applied to the second examination region. This application is well known and applies a compensating sequence control pulse rather than a predefined sequence control pulse.
The solution of the invention allows an active compensation of stray gradient fields at least in the first order. By this compensation, images in different examination zones can be acquired simultaneously and independently, wherein in each examination zone a dedicated triaxial gradient system can be operated. If the target field of view is not too large and the active shielding of the gradient coils is reasonably effective, the compensation of stray fields up to the first order is good enough. However, this method can be extended to correct for higher order stray fields. This would preferably require a set of dynamic higher order shimming coils and associated coil current amplifiers and correspondingly a larger sensitivity matrix for inversion. The higher order compensation is further described below.
Although the invention is very advantageous for a star-shaped magnet arrangement, other MRT systems are also advantageous for a linear arrangement of the examination zone or an arrangement of the "satellite examination zone" with, for example, a basic magnetic field using a central examination zone.
The system according to the invention for compensating for stray magnetic fields in a magnetic resonance imaging system having two or more examination zones is particularly designed to perform the method according to the invention. The system comprises the following components:
a data interface for receiving values of a predefined first magnetic field to be applied in the first examination zone in addition to the basic magnetic field and for receiving information defining a predefined sequence of control pulses to be applied in the second examination zone. Such data interfaces are well known and are preferably designed to read a data memory or to communicate via a data network.
A determination unit designed to determine a stray magnetic field in the second examination zone for the case of application of the first magnetic field in the first examination zone. The determination unit may be designed to calculate the stray magnetic field from information about the respective MRI scanner. However, it may also comprise a sensor unit to measure stray magnetic fields in the respective examination zone. The determination unit may also be designated as a "stray field determination unit".
A calculation unit designed to calculate a compensation sequence control pulse for the second examination zone from the predefined sequence control pulse and the determined stray magnetic field. The calculation unit may also be designated as "compensation unit".
An application unit designed to apply a compensation sequence control pulse to the second examination region. The application unit may be a data interface for transmitting data of the compensation sequence control pulses to a control unit of the MRI scanner. However, it may also comprise units capable of directly driving the coils or antennas of the MRI system (i.e. applying a current to these coils).
The control device for controlling a magnetic resonance imaging system according to the invention comprises a system according to the invention. Alternatively or additionally, it is designed to carry out the method according to the invention. The control device may comprise additional units or devices for controlling components of the magnetic resonance imaging system, for example a sequence control unit for measurement sequence control, a memory, a radio frequency transmission device for generating, amplifying and transmitting RF pulses, a gradient system interface, a radio frequency reception device for obtaining magnetic resonance signals and/or a reconstruction unit for reconstructing magnetic resonance image data.
A magnetic resonance imaging system comprises two or more examination zones and a control device according to the invention. A preferred MRI scanner of such a magnetic resonance imaging system comprises an inclined arrangement, e.g. a star arrangement, of basic field magnets. MRI scanners with a ring MRI scanner architecture are particularly preferred.
Some of the units or modules of the above mentioned system or control device may be implemented fully or partly as software modules running on a processor of the system or control device. An implementation mainly in the form of software modules may have the advantage that applications already installed on an existing system may be updated with relatively little effort to install and run the units of the present application. The object of the invention is also achieved by a computer program product with a computer program directly loadable into a memory of a device of the system or of a control device of the magnetic resonance imaging system and comprising program elements for performing the steps of the method of the invention when the program is executed by the control device or system. In addition to computer programs, such computer program products may include other parts such as documents and/or additional components, as well as hardware components such as hardware keys (dongles, etc.) to facilitate access to the software. It should be noted that the application of the sequence control pulses is first of all the action "sending corresponding data about the amplitude of the pulses to the amplifier unit". This may be performed by a computing unit having a data interface.
A computer-readable medium, such as a memory stick, hard disk, or other removable or permanently mounted carrier, may be used to transfer and/or store executable portions of the computer program product such that these portions can be read from a processor unit of a control device or system. The processor unit may include one or more microprocessors or equivalents thereof.
Particularly advantageous embodiments and features of the invention are given by the dependent claims, as disclosed in the following description. Features from different claim categories may be combined as appropriate to give other embodiments not described herein.
The preferred method is applicable when the sequence control pulse is a second magnetic field. The method comprises the following steps:
in addition to the value for the predefined first magnetic field, a value for a predefined second magnetic field to be applied in a second examination zone, in particular a zone adjacent to the first examination zone, is provided (i.e. a sequence control pulse). Preferably, the first magnetic field and the second magnetic field are both gradient fields. However, to compensate for the shift in the RF readout, the second magnetic field may also be the B0 field (e.g., a gradient field) that is affected by the first magnetic field.
The stray field of the first magnetic field in the second examination zone is then determined as described above.
-after determining the stray magnetic field in the second examination zone, calculating a compensation magnetic field for the second examination zone from the predefined second magnetic field and the determined stray magnetic field. This is achieved in particular by adding or subtracting the vector of the stray magnetic field to or from the vector of the first magnetic field. However, it is also possible to calculate a correction field and add/subtract it to/from the second magnetic field (depending on the direction). Technically, it is preferred to calculate the compensation current that has to be applied, rather than the predefined current in the predefined coil (of the predefined second magnetic field).
-applying a compensation magnetic field to the second examination zone. Preferably, this is done for all gradients Gx, Gy and Gz, preferably performed at each examination region. Technically, this is preferably achieved by applying the above-mentioned compensation current to a predefined coil. It should be noted that "current" can be read, rather than "magnetic field", because current generates a magnetic field and directly defines the field.
The preferred method is applicable when the sequence control pulse is an RF signal. The method may alternatively or additionally be applied to the method for magnetic fields described previously. It should be noted that stray fields leaking into the examination area shift the effective basic magnetic field B0 by a fraction Δ B0 and thus also the Lamour frequency by the offset Δ f 0.
The preferred method comprises the steps of:
-providing, in addition to the value of the predefined first magnetic field, a value of the frequency (f0) of the predefined RF signal (i.e. here the sequence control pulse) to be applied in the second examination zone, in particular the zone adjacent to the first examination zone.
-after determining the stray magnetic field in the second examination zone, calculating a compensated RF signal for the second examination zone from the predefined RF signal and the determined stray magnetic field. This is achieved in particular by calculating the compensation frequency Δ f0 (i.e. the shift of the Lamour frequency in the second examination zone due to the influence of stray magnetic fields) and calculating the compensation frequency from the predefined frequency f0 and the compensation frequency Δ f 0.
-applying the compensated RF signal to the second examination region. This is performed in particular by: the leakage from all other gradient coils was incorporated into the first correction of the Larmor frequency to an offset Δ f0, which corresponds to the average offset in the static magnetic field Δ B0. This frequency correction is preferably effected by digitally adjusting the frequency offset of the clock synthesizer for each examination zone.
According to a preferred method, a magnetic resonance system with a gradient system having a plurality of M examination zones and L axes in each examination zone is calculated with a coefficient km,nThe field shift matrix. Typically, L equals 3 for the X-, Y-, and Z-axes. The field shift matrix may be measured or provided (calculated or read from a data store). For the index, M is from 1 to M and denotes the examination area, n is from 1 to L × M and denotes the gradient axis of the different examination areas. For example, n-1 means the X-gradient axis in the first examination region, n-2 means the Y-gradient axis in the first examination region, n-3 means the Z-gradient axis in the first examination region, n-4 means the X-gradient axis in the second examination region, and so on.
Based on formula km,n＝g(ΔB0m)nAccording to the time-dependent field variation Δ B0 in the isocenter of each examination region mmBy a function ofm,nWherein g () is preferably linearly dependent on Δ B0mAs a function of (c). Preferably, the field-shift coefficient k is pre-calculated by using data from the gradient coil designm,nOr the field shift coefficient k is obtained by a calibration testm,n。
And then based on the field shift matrix (i.e., according to the coefficient k)m,n) To calculate a compensation sequence control pulse for the examination area m. Preferably, the compensation frequency Δ f0 for compensating the RF signals of the examination region m is calculated on the basis of a field shift matrix according to the following formulam
Figure BDA0002798896720000071
Wherein γ is a gyromagnetic constant, and g (k)m,n) Is km,nThereby producing a magnetic flux density.
Preferably, the gradient coils C according to a gradient systemnCurrent I innBy km,n＝ΔB0m/InCalculating coefficient k of field shift matrixm,n(i.e., g (k)m,n)＝ΔB0m/In). Therefore, the current I in the magnetic coil according to the examination zone is based on the following formulanAnd a field shift matrix to calculate a compensation frequency Δ f0 for compensating the RF signals of the examination region mm
Figure BDA0002798896720000081
For example, for a B0 correction, the correction can be made by contributing all stray fields of the gradient coil (by the current I in all other active gradient coils)nKnown or measured) to calculate a time-dependent field variation within the isocenter of the examination region m. Typically, there will be three field shift coefficients between the examination regions, requiring more than 18 for a scanner with six examination regionsThe sum of the terms. It should be noted that km,nSome of the terms may be zero (i.e., from term k)m,n) But some cross terms may also approach zero by symmetry. For example, any Gy gradient coil will produce little B0 displacement to any other examination region.
According to a preferred method, the sequence control pulses are gradient signals and a sensitivity matrix S is created comprising the contributions of each gradient field to each examination region. A compensated gradient field of the gradient axis in the examination region is calculated on the basis of the sensitivity matrix S.
In the preferred case where there are P gradient coils, the matrix S comprises P × P coefficients Sp,nWhere both P and n are from 1 to P, in particular where P is L × M for a gradient system of a plurality of M examination zones and L axes (preferably 3) in each examination zone. It should be noted that each gradient system should have its own local coordinate system, which is adjusted to the respective examination region.
Preferably, the coefficients S of the sensitivity matrix Sp,nCorresponding to the gradient fields effective in the respective examination region with respect to the (local) axis of the gradient system and are calculated, measured or provided.
It is also preferred that at current InFlows through a gradient coil Cn(i.e. for the current InOf the sensitivity matrix S) comprises the values of the magnetic gradient fields applied in the axes of the gradient system in the respective examination region, preferably wherein the coefficient Sp,nAnd the gradient field value GpDivided by passing through a gradient coil CnCurrent of (I)nAnd (7) corresponding. It should be noted here that each current flows through only one well-defined coil (each current flows through the other coil). The current for the Z-gradient in the examination zone 1 should flow through the coils or the like for the Z-gradient in this examination zone 1. However, the current flowing through the coil is in the respective examination region (i.e. s)n，n) A magnetic field is induced and in the other examination zone (i.e. s)p,nWhere p ≠ n), where the stray field is strongest in the neighboring examination area. Thus, in any column of matrix SIn the line, there should be one maximum coefficient, where the neighboring coefficients represent the stray field. The other coefficients are typically zero or close to zero.
Preferably, for the gradient coils C of the gradient systemnOf a predefined gradient value GnCalculating the value to be applied to the gradient coil C from the sensitivity matrix SnCurrent of (I)n. This is preferably done by using predefined gradient values G comprising axes for the gradient system in the respective examination regionpIs completed by the predefined gradient vector G. Then, based on the formula I ═ S-1 · G, from the inverse sensitivity matrix S-1To calculate the gradient coil C to be applied tonComprises a (compensation) current InThe vector of (a). Thus, the vector G filled with predefined values for the gradient is corrected by the sensitivity matrix S, and the resulting vector I comprises the currents that have to be applied to obtain the correct gradient field, regardless of the stray field. It should be understood here that the gradient and current waveforms are preferably both functions of time.
For example, there is a gradient sensitivity matrix S Sp,n＝Gp/InWhere p, n 1.. 3 × M (for M examination regions each having three gradient axes) describes the (initial) gradient current InThe spatial derivatives of the stray field around the isocenter in all three directions of the local coordinate system are induced. In this example, S is an 18 x 18 matrix for 6 examination regions, which actually contains some zero or near-zero entries off the diagonal. Now a correction should be applied to the m-th gradient current to eliminate unnecessary contributions from all other gradient currents, which will again change the stray field in all other examination regions. The correction typically needs to be calculated iteratively. A straightforward and non-iterative solution is to invert the gradient sensitivity matrix after calibration (see above). Then, to achieve the target gradient waveform of 6 × 3 gradient, S is given by the above formula I-1G simply calculates the waveform vector required for the 18 gradient currents, where G is a matrix of the 18 target gradients to be achieved within the examination region.
According to a preferred method, the compensation sequence control pulses are iteratively calculated by calculating a compensated first magnetic field in the first examination zone based on the stray magnetic field of the compensation sequence control pulses in the first examination zone. If a correction is applied to the m-th gradient current to eliminate unnecessary contributions from all other gradient currents, this will again change the stray field in all other examination regions. Therefore, it is preferable to increase the correction of the iterative computation.
The preferred system is designed to calculate and apply compensation sequence control pulses in the form of gradient fields and/or RF signals.
Drawings
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for the purposes of illustration and not as a definition of the limits of the invention.
Fig. 1 shows a simplified MRI system comprising a system according to a preferred embodiment of the present invention.
Fig. 2 shows an exemplary embodiment of a magnetic resonance tomography system with a star-shaped basic field magnet arrangement.
Figure 3 shows another exemplary embodiment of a magnetic resonance scanner with two examination zones.
Fig. 4 shows an example of a stray field distribution.
Fig. 5 shows a block diagram of the process flow of a preferred method according to the invention.
In the drawings, like reference numerals refer to like elements throughout. The objects in the drawings are not necessarily to scale.
Detailed Description
Fig. 1 shows a schematic view of a magnetic resonance imaging system 1 ("MRI system"). The MRI system 1 comprises a real magnetic resonance scanner (data acquisition unit) 2 with an examination space 3 or a patient tunnel, in which a patient or a test person is located on a drive couch 8 in the examination space 3 or patient tunnel, in which a real examination object O is located.
The magnetic resonance scanner 2 is typically equipped with a basic field magnet system 4, a gradient system 6, and an RF transmit antenna system 5 and an RF receive antenna system 7. In the exemplary embodiment shown, the RF transmit antenna system 5 is a whole-body coil permanently mounted in the magnetic resonance scanner 2, in contrast to which the RF receive antenna system 7 is formed as a local coil (here represented by only a single local coil) to be arranged on the patient or test object. In principle, however, the whole-body coil can also be used as an RF receiving antenna system, and the local coils can each be switched to a different operating mode.
The basic field magnet system is typically designed such that it generates a basic magnetic field in the longitudinal direction of the patient (i.e. running in the z-direction along the longitudinal axis of the magnetic resonance scanner 2). The gradient system 6 typically comprises individually controllable gradient coils in order to be able to switch (activate) the gradients in the x-direction, the y-direction or the z-direction independently of one another.
The MRI system 1 shown here is a whole-body system with a patient tunnel into which the patient can be introduced completely. However, in principle the invention may also be used for other MRI systems, such as C-shaped housings with a lateral opening, and smaller magnetic resonance scanners in which only one body part can be placed.
Furthermore, the MRI system 1 has a central control device 13 for controlling the MRI system 1. The central control device 13 comprises a sequence control unit 14 for measurement sequence control. With the sequence control unit 14, the sequence of radio frequency pulses (RF pulses) and gradient pulses can be controlled in accordance with the selected pulse sequence.
For outputting the individual RF pulses of the pulse sequence, the central control device 13 has a radio frequency transmission device 15, which radio frequency transmission device 15 generates and amplifies the RF pulses and feeds them into the RF transmission antenna system 5 via a suitable interface (not shown in detail). For controlling the gradient coils of the gradient system 6, the control device 13 has a gradient system interface 16. The sequence control unit 14 communicates in a suitable manner with the radio frequency transmission device 15 and the gradient system interface 16 to transmit the pulse sequence.
Furthermore, in order to obtain the magnetic resonance signals (i.e. the raw data) for the individual measurements, the control device 13 has a radio frequency receiving device 17 (which likewise communicates in a suitable manner with the sequence control unit 14) which receives the magnetic resonance signals from the RF receiving antenna system 7 in a coordinated manner within the scope of the pulse sequence.
A reconstruction unit 18 receives the obtained raw data and reconstructs magnetic resonance image data therefrom for measurement. The reconstruction is typically performed based on parameters that may be specified in the respective measurement or control protocol. The image data may then be stored in the memory 19, for example.
The operation of the central control device 13 may be performed via a terminal 10 having an input unit and a display unit 9, and therefore, the entire MRI system 1 may also be operated by an operator via the terminal 10. The MR image may also be displayed on the display unit 9 and the measurement may be planned and started by means of the input unit (possibly in combination with the display unit 9), and in particular a suitable control protocol may be selected (and possibly modified) using a suitable series of pulse sequences PS as described above.
The MRI system 1 according to the invention, and in particular the control device 13, may have a number of additional components, not shown in detail but usually present in such systems, such as a network interface, in order to connect the entire system with a network and to be able to exchange raw data and/or image data or parameter maps, respectively, but also other data (e.g. patient-related data or control protocols, etc.).
The manner in which suitable raw data are obtained by transmission of RF pulses and generation of gradient fields and MR images are reconstructed from the raw data is known to the person skilled in the art and therefore need not be explained in detail herein.
Fig. 2 shows an exemplary embodiment of a magnetic resonance tomography system 1 with a star-shaped basic field magnet arrangement 40.
Here, a magnetic resonance scanner 2 is shown, the function of which can be controlled by a control device 13. The control device 13 can in principle be constructed in a similar manner and have the same components as the control device 13 in the conventional MR system according to fig. 1. Also it may have suitable terminals etc. (not shown here).
In this example, the control unit 13 comprises a system 12 for compensating for stray magnetic fields in a magnetic resonance imaging system according to the invention having two or more examination zones. With respect to the pulses and fields, this is shown in FIG. 4 and described below. The system 12 includes:
a data interface 20 for receiving values of a predefined first magnetic field to be applied in a first examination zone M1, M2, M3, M4, M5, M6 (each examination zone may serve as a first examination zone) in addition to the basic magnetic field B0, and for receiving information defining a predefined sequence of control pulses to be applied in a second examination zone M1, M2, M3, M4, M5, M6 (any other examination zone in addition to the first examination zone may serve as a second examination zone).
A determination unit 21 which is designed to determine the stray magnetic field SF in the second examination zones M1, M2, M3, M4, M5, M6 for the case of an application of a first magnetic field in the first examination zones M1, M2, M3, M4, M5, M6.
A calculation unit 22 which is designed to calculate compensation sequence control pulses for the second examination zones M1, M2, M3, M4, M5, M6 from the predefined sequence control pulses and the determined magnetic stray field SF.
An application unit 23, which is designed to apply compensation sequence control pulses to the second examination zones M1, M2, M3, M4, M5, M6.
The basic field magnet arrangement 40 of the magnetic resonance scanner 2 in this figure comprises six (here identical) basic field magnet segments 44, which in this embodiment are arranged in a star shape around the central axis a with a rotational symmetry of 60 °. The basic magnetic field B0 indicated by an arrow has a basic field main direction R0, which extends in the form of a circular magnetic field or a ring-shaped magnetic field.
In both examination regions, the local coordinate systems of the local gradient fields generated by the gradient system 6 are shown (in fig. 3, only two gradient systems 6 are shown in the front examination region M4 and the right-front examination region M3. of course, all examination regions M1, M2, M3, M4, M5, M6 may comprise gradient systems). The y-axis in this example always points upwards, the z-axis follows the direction of the basic magnetic field B0 (at least at the isocenter), and the x-axis points outwards (at least from the isocenter) perpendicular to the basic magnetic field B0. This solution of a local coordinate system can be applied to all examination zones M1, M2, M3, M4, M5, M6, so that the x-axis and z-axis of the local coordinate system are always different.
Fig. 3 shows a further exemplary embodiment of a magnetic resonance scanner 2 with two examination zones M1, M2. Here, only the lower half of the basic field magnet arrangement 40 is designed in the shape of a star as a group 41 of basic field magnet segments 44, and the other basic field magnet segment 44 protrudes upwards, and both serve to guide the basic magnetic field B0 and the part of the wall W between the two examination zones M1, M2 on which two patients are present as objects O to be examined. In the figure it can be seen that the lower part of the wall W between the two patients is formed by the housing wall 30 of the magnetic resonance scanner 2, into which housing wall 30 the basic field magnetic section 44 is integrated between the examination zones M1, M2. The wall W may not only serve as a privacy screen but also as an acoustic or RF shield.
The basic magnetic field B0 of the magnetic resonance scanner 2 becomes weaker towards the outside, can be used for position coding, and is uniform in the longitudinal direction (orthogonal to the image plane P). The shape is substantially the same in both examination zones M1, M2, wherein the only difference is that the course (in one direction across the surface on which the patient O lies) is reversed. Also the dimensions of the magnetic resonance scanner 2 can be chosen to be completely different.
Here, the basic main magnetic field direction R0 is also circular. A special feature of this embodiment is that the patient O is not in a narrow space but can freely look towards the ceiling. As described above, inhomogeneities in the basic magnetic field B0, which are usually caused by curvature, can be used for spatial encoding resolution in one spatial direction, so that for the total spatial resolution the total spatial encoding applies gradients in only the other spatial direction.
This arrangement allows easy and almost unrestricted access to the patient due to its open design and the toroidal magnetic field. Due to the special construction, the magnetic force is largely compensated as shown in fig. 2, or is transferred to a region which can be structurally well enhanced.
An example of a gradient system 6 is shown in two examination zones M1, M2. The V-shape of the gradient system 6 again follows the angle between the two basic field magnets 44, i.e. here 90 °.
Fig. 4 shows an example of the distribution of the stray field SF and the magnetic field. By activated Gz gradient fields GZ1The resulting stray gradient fields leak into the region. The Z-axis of the local coordinate system extends from left to right (see e.g. fig. 2). For example, in a star arrangement of magnets, the Z-axes of all coordinate systems extend in the shape of a circle or polygon. The Z axis is shown here as a straight line, wherein the examination zones M1, M2 lie on a line adjacent to one another. The vertical dashed lines shall indicate the boundaries of the examination zones M1, M2 (and also the basic field magnets). Shown as vertical boxes are coils C1, C2 for applying a Z-gradient in the examination zones M1, M2. In a first examination zone M1, a Z gradient field G is appliedZ1(i.e., the first magnetic field). The intensity of the field is shown in the image plane as a distance from the Z-axis (the x-axis and the y-axis of the coordinate system are not shown here).
It can be seen here that the first magnetic field (here the solid line) does not disappear outside the boundaries of the first examination zone M1, but rather forms a stray field SF. Even if the primary gradient coils in region 1 are actively shielded (for example, if the box would also act as a magnet shield), the stray magnetic field would be orders of magnitude smaller, but the stray magnetic field SF would remain (dashed line expanded in the second examination region M2). Due to field disturbances, the gradient field G applied as a second magnetic field in the second examination zone M2Z2(dot-dash line) will be affected by the stray field SF.
To compensate for the effects of the stray field SF, the present invention determines the effects of the stray field SF and modifies the applied magnetic field (i.e., the compensation sequence control pulse) so that the resulting field is the second magnetic field despite the presence of the stray field SF. To achieve this, it is preferred to calculate the difference gradient field Δ G between the stray magnetic field SF and the predefined second magnetic field, as shown in the figureZ2。
With respect to the unshielded first magnetic field GZ1With a stray field SF larger than a predefined second magnetic field GZ2. Therefore, a cancellation field must be applied. Here, the compensation sequence control pulse is relative to a predetermined gradient field GZ2Applied differential gradient field Δ GZ2. First magnetic field G about the shieldZ1With stray field SF smaller than a predetermined second magnetic field GZ2. Therefore, a second magnetic field G which is more predefined than the predefined one has to be appliedZ2A weak compensation field. Here, the compensation sequence control pulse is a predefined gradient field GZ2Minus the gradient field Δ GZ2。
Fig. 5 shows a block diagram of a process flow of a preferred method according to the invention for compensating for a stray magnetic field SF in a magnetic resonance imaging system 1 (e.g. fig. 2) having two or more examination zones M1, M2, M3, M4, M5, M6. For visualization of the field see, for example, fig. 4.
In step I, in addition to the basic magnetic field B0, a predefined first magnetic field G is applied in the first examination zone M1Z1A value is provided.
In step II, a predefined sequence of control pulses G is provided which defines the second examination zone M2 to be appliedZ2The information of (1). In fig. 4, the predefined sequence controls the pulse GZ2Is a gradient field GZ2However, it may also be an RF signal.
In step III, for the application of a first magnetic field G in a first examination zone M1Z1Determines the magnetic stray field SF in the second examination zone M2.
In step IV, the pulse G is controlled according to a predefined sequenceZ2And the determined magnetic stray field SF, to calculate a compensation sequence control pulse CG for the second examination zone M2.
In step V, the compensation sequence control pulse CG is applied to the second examination region M2.
Although the present invention has been disclosed in the form of preferred embodiments and variations thereon, it will be understood that numerous additional variations and modifications may be made thereto without departing from the scope of the invention. For the sake of clarity, it is to be understood that the use of "a" or "an" throughout this application does not exclude a plurality, and "comprising" does not exclude other steps or elements. Reference to "a unit" or "a device" does not exclude the use of more than one unit or device.

Claims (15)

1. A method for compensating for stray magnetic fields (SF) in a magnetic resonance imaging system (1) having two or more examination zones (M1, M2, M3, M4, M5, M6), comprising the steps of:
-providing, in addition to the basic magnetic field (B0), a predefined first magnetic field (G) to be applied in the first examination zone (M1)Z1) The value of (a) is,
-providing a predefined sequence of control pulses (G) defining to be applied in the second examination zone (M2)Z2) Is determined by the information of (a) a,
-for applying the first magnetic field (G) in the first examination zone (M1)Z1) Determining a stray magnetic field (SF) in the second examination zone (M2),
-controlling pulses (G) according to said predefined sequenceZ2) And the determined stray magnetic field (SF) calculating a compensation sequence control pulse (CG) for the second examination zone (M2),
-applying the compensation sequence control pulse (CG) to the second examination region (M2).
2. The method according to claim 1, wherein the sequence of control pulses is a second magnetic field (G)Z2) The method comprises the following steps:
-in addition to said predefined first magnetic field (G)Z1) In addition to the value of (c), a predefined second magnetic field (G2) to be applied in a second examination zone (M2) is also providedZ2) The value of (a) is,
-after determination of a magnetic Stray Field (SF) in the second examination zone (M2), according to the predefined second magnetic field (G)Z2) And the determined magnetic Stray Field (SF) calculating a compensation magnetic field (CG) for the second examination zone (M2),
-applying the compensation magnetic field (CG) to the second examination zone (M2).
3. The method according to one of the preceding claims, wherein the sequence of control pulses is an RF signal, comprising the steps of:
-in addition to said predefined first magnetic field (G)Z1) In addition to the value of (c), also provides a value of the frequency of the predefined RF signal to be applied in the second examination zone (M2),
-after determination of the magnetic Stray Field (SF) in the second examination zone (M2), calculating a compensated RF signal for the second examination zone (M2) from the predefined RF signal and the determined magnetic Stray Field (SF),
-applying the compensated RF signal to the second examination region (M2).
4. Method of one of the preceding claims, wherein for a magnetic resonance system (1) with a gradient system (6) of a plurality of L axes in the M examination region (M1, M2, M3, M4, M5, M6) and each examination region (M1, M2, M3, M4, M5, M6), based on the formula km,n＝g(ΔB0m)nAccording to the time-dependent field change Δ B0 in the isocenter of each examination region M (M1, M2, M3, M4, M5, M6)mIs calculated, measured or provided with a coefficient km,nWherein M is (1.. M) and n is (1.. lxm), and a compensation sequence control pulse (CG) for the examination region M (M1, M2, M3, M4, M5, M6) is calculated based on the field shift matrix,
preferably, wherein, according to the formula
Figure FDA0002798896710000021
Calculating a compensation frequency Δ f0 for compensating RF signals of the inspection region M (M1, M2, M3, M4, M5, M6) based on the field shift matrixm，
Wherein γ is a gyromagnetic constant, and g (k)m,n) Is km,nThereby producing a magnetic flux density.
5. The method of claim 4, wherein k is a function of km,n＝ΔB0m/InGradient coil C of the gradient system (6)nCurrent I in (C1, C2)nTo calculate said field shift matrixCoefficient km,n，
And wherein, according to the formula
Figure FDA0002798896710000022
Calculating a compensation frequency Δ f0 for compensating RF signals of the inspection region M (M1, M2, M3, M4, M5, M6) based on the field shift matrixm。
6. Method according to one of the preceding claims, wherein the sequence control pulse is a gradient signal (G)Z2) And wherein a sensitivity matrix S is created which comprises the contribution of each gradient field to each examination zone (M1, M2, M3, M4, M5, M6), and wherein a compensating gradient field of the gradient axis in the examination zone (M1, M2, M3, M4, M5, M6) is calculated on the basis of the sensitivity matrix S,
preferably, wherein for P gradient coils (C1, C2), the matrix S comprises P coefficients Sp,nWherein P and n are both from 1 to P, in particular wherein P is L × M for a gradient system (6) of L axes in a plurality of M examination zones (M1, M2, M3, M4, M5, M6) and each examination zone (M1, M2, M3, M4, M5, M6).
7. The method of claim 6, wherein the coefficients S of the sensitivity matrix Sp,nCorresponding to the gradient fields effective in the respective examination zone (M1, M2, M3, M4, M5, M6) relative to the axis of the gradient system (6) and calculated, measured or provided,
preferably wherein, at the current InFlows through a gradient coil Cn(C1, C2), the rows or columns of the sensitivity matrix S comprise the values of the magnetic gradient fields applied in the axes of the gradient system (6) in the respective examination zone (M1, M2, M3, M4, M5, M6),
preferably wherein said coefficient sp,nAnd the gradient field value GpDivided by passing through a gradient coil CnCurrent I of (C1, C2)nAnd (7) corresponding.
8. The method of claim 6 or 7, wherein for a gradient coil C of the gradient system (6)n(C1, C2) of a predefined gradient value GnCalculating, from the sensitivity matrix S, a value to be applied to the gradient coil CnCurrent I of (C1, C2)n，
Preferably wherein, for a predefined gradient vector G of predefined gradient values Gp of the axes of the gradient system (6) comprised in the respective examination zone (M1, M2, M3, M4, M5, M6), according to the inverse sensitivity matrix S, based on the formula I-S-1 · G-1To calculate a gradient value including a gradient value to be applied to the gradient coil CnCurrent I of (C1, C2)nThe vector of (a).
9. Method of one of the preceding claims, wherein the compensation sequence control pulse (CG) is calculated iteratively by calculating a compensated first magnetic field in the first examination zone (M1) based on a stray magnetic field (SF) in the first examination zone (M1) of a compensation sequence control pulse (CG) in the second examination zone (M2).
10. A system (12) for compensating for stray magnetic fields (SF) in a magnetic resonance imaging system (1) having two or more examination zones (M1, M2, M3, M4, M5, M6), in particular by performing the method according to one of the preceding claims, the system (12) comprising:
a data interface (20) for receiving a predefined first magnetic field (G) which is applied in addition to the basic magnetic field (B0) in the first examination zone (M1)Z1) And a defined predefined sequence of control pulses (G) to be applied in the second examination zone (M2)Z2) Is determined by the information of (a) a,
-a determination unit (21) designed to apply the first magnetic field (G) in the first examination zone (M1)Z1) Determining a stray magnetic field (SF) in the second examination zone (M2),
-a calculation unit (23) designed to control the pulses (G) according to said predefined sequenceZ2) Andthe determined stray magnetic field (SF) is used to calculate a compensation sequence control pulse (CG) for the second examination zone (M2),
-an application unit (24) designed to apply the compensation sequence control pulses (CG) to the second examination region (M2).
11. The system according to claim 10, which is designed to calculate and apply compensation sequence control pulses in the form of gradient fields and/or RF signals.
12. A control device (13) for controlling a magnetic resonance imaging system (1), the control device (13) comprising a system according to claim 10 or 11 or being designed to perform a method according to one of the claims 1 to 9.
13. A magnetic resonance imaging system (1) comprising a control device (13) according to claim 12.
14. A computer program product comprising a computer program directly loadable into a system (12) or a control device (13) for a magnetic resonance imaging system (1), the computer program product comprising program elements for performing the steps of the method as claimed in any of claims 1 to 9, when the computer program is executed by the system (12) or the control device (13).
15. A computer-readable medium, having stored thereon a program element, which is readable and executable by a computer unit for performing the steps of the method according to any one of claims 1 to 9, when the program element is executed by the computer unit.
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